Method for the decomposition of ammonium formates in reaction mixtures containing polyol

ABSTRACT

The invention relates to a method for producing polymethylol compounds of formula (I), in which R represents a further methylol group or an alkyl group with between 1 and 22 C atoms or an aryl or alkyl aryl group with between 6 and 22 C atoms, by condensating aldehydes with between 2 and 24 C atoms with formaldehyde, using tertiary amines as catalysts, with a view to obtaining compounds of formula (II), in which R has the meaning assigned to it above. The invention further relates to a process for hydrogenating polymethylol compounds by condensation in stages. For this, a) in a first (reaction) stage, the aldehydes with 2 or more C atoms are reacted with 2 to 8 times the molar quantity of formaldehyde in the presence of a tertiary amine as catalyst; b) in a second (separation) stage, either the reaction mixture is separated into a distillation residue containing primarily the compounds of formula (II) and a destillation flow containing primarily the compounds of unreacted or partly reacted starting materials which is fed back to the first stage, or the reaction mixture of the first stage is separated into an aqueous and an organic phase by means of a phase separation device and the organic phase fed back to the first stage, and c) in a third (post-reaction distillation) stage, the distillation residue of the second stage or the aqueous phase obtained by phase separation during the second stage is subjected to a catalytic and/or heat treatment, the compound of formula (III) which has not been fully methylolized is transformed into the corresponding compound of formula (II) and into the corresponding methylene compound of formula (IV), in which R′ represents hydrogen or has the meaning assigned above to R, the reaction mixture obtained in this way is distilled, and the distillation overhead product, which contains a compound of formula (IV) and unreacted formaldehyde, is fed back to the first stage. Thereafter, the still bottom of this distillation process, which essentially contains the compound of formula (II), is hydrogenated in the known manner so as to obtain the corresponding end product of formula (I).

[0001] The invention relates to the field of industrial organicchemistry. More precisely, the present invention provides a process forthe effective decomposition of trialkylammonium formate which is presentin methylolalkanes and has been formed from the trialkylamine used ascatalyst in the preparation of the methylolalkanal and the formic acidformed as by-product.

[0002] The condensation of formaldehyde with CH-acid higher alkanals toform methylolalkanals, in general dimethylolalkanals andtrimethylolalkanals, and conversion of the compounds obtained intopolyols is a widely employed process in industrial chemistry. Examplesof important triols obtained in this way are trimethylolpropane,trimethylolethane and trimethylolbutane, which have found widespread usein the production of surface coatings, urethanes and polyesters. Furtherimportant compounds are pentaerythritol, obtainable by condensation offormaldehyde and acetaldehyde, and also neopentyl glycol fromisobutyraladehyde and formaldehyde. The tetravalent alcoholpentaerythritol is likewise frequently used in the surface coatingsindustry, but has also achieved great importance in the production ofexplosives.

[0003] The polyols mentioned can be prepared by various methods. Onemethod is the Cannizzaro process which is further subdivided into theinorganic Cannizzaro process and the organic Cannizzaro process. In theinorganic variant, an excess of formaldehyde is reacted with thecorresponding alkanal in the presence of stoichiometric amounts of aninorganic base such as NaOH or Ca(OH)₂. The methylolalkanal formed inthe first step reacts in the second step with the excess formaldehyde ina disproportionation reaction to form the corresponding polyol and theformate of the respective base, i.e., for example, sodium or calciumformate.

[0004] In the organic Cannizzaro process, a tertiary amine, generally atrialkylamine, is used in place of the inorganic base. The reactionproceeds as described above, with one equivalent of the ammonium formateof the corresponding amine being formed. This can be worked up furtherby appropriate methods, so that at least the amine can be recovered andreturn to the reaction. The crude polyol obtained can be worked up invarious ways to give the pure polyol.

[0005] A further development is the hydrogenation process in which anappropriate alkanal and formaldehyde are reacted with one another not inthe presence of at least stoichiometric amounts but of catalytic amountsof a tertiary amine, generally from about 5 to 10 mol %. In thisprocess, the reaction stops at the stage of 2,2-dimethylolalkanal whichis subsequently converted into trimethylolalkane by hydrogenation. Adescription of the effective process may be found in WO 98/28253 of thepresent applicant.

[0006] A number of variants of this hydrogenation process are described,inter alia, in the patent applications DE-A-25 07 461, DE-A-27 02 582,DE-A-28 13 201 and DE-A-33 40 791.

[0007] Although the hydrogenation process advantageously does not formstoichiometric amounts of the formate as in the organic Cannizzaroprocess, trialkylammonium formate is formed as product of across-Cannizzaro reaction occurring to a small extent as secondaryreaction.

[0008] Trialkylammonium formates react under particular conditions, forexample, the dewatering or heating of trimethylolalkane solutionsobtained, to form trialkylamine and trimethylolpropane formate. Thesedecrease the yield of trimethylolalkane and are difficult to dissociatewithout undesirable degradation reactions. There is therefore particularinterest in the removal of trialkylammonium formates.

[0009] DE 198 48 569 discloses a process for the decomposition offormates of tertiary amines which are present as by-products intrimethylolalkane solutions prepared by the organic Cannizzaro process.These formates are decomposed by heating, preferably in the presence ofmodified noble metal catalysts and under superatmospheric pressure, intohydrogen and carbon dioxide and/or water and carbon monoxide and thetertiary amine. The formate conversions in this process areunsatisfactory, and the formation of further by-products is alsoobserved.

[0010] In addition, the abovementioned process has only limitedsuitability for the effective work-up of a trimethylolalkane mixtureobtained by the hydrogenation process in which only catalytic amounts oftrialkylamine are used and the product mixture thus also contains onlysmall amounts of trialkyammonium formate.

[0011] It is an object of the present invention to provide a processwhich is suitable for the work-up of reaction mixtures obtained by thehydrogenation process and also those obtained by the organic Cannizzaroprocess. Furthermore, this process should make it possible to preparetrimethylolalkanes having a high purity and a low color number,preferably less than 10 Apha.

[0012] We have found that this object is achieved by a process forremoving trialkylammonium formate from methylolalkanes which have beenobtained by condensation of formaldehyde with a higher aldehyde, whichprocess comprises decomposing trialkylammonium formate at elevatedtemperature in the presence of a hydrogen-containing gas over catalystscomprising at least one metal of groups 8 to 12 of the Periodic Table.

[0013] Methylolalkanes which can be worked up by the process of thepresent invention are, for example, neopentyl glycol, pentaerythritol,trimethylolpropane, trimethylolbutane, trimethylolethane,2-ethyl-1,3-propanediol, 2-methyl-1,3-propane-diol, glycerol,dimethylolpropane, dipentaerythritol and 1,1-, 1,2-, 1,3- and1,4-cyclohexanedimethanol.

[0014] In the process of the present invention, preference is given toremoving trialkylammonium formates from trimethylolalkanes which havebeen prepared by the organic Cannizzaro process or the hydrogenationprocess. Preference is given to purifying trimethylolalkanes,particularly preferably trimethylolpropane, hereinafter referred to asTMP for short, prepared by the hydrogenation process.

[0015] The preparation of crude TMP containing trialkylammonium formateby the Cannizzaro process is disclosed, for example, in DE 198 48 569.

[0016] In the hydrogenation process, the TMP is obtained by condensationof n-butyraldehyde with formaldehyde in the presence of catalyticamounts of a tertiary amine and subsequent catalytic hydrogenation ofthe dimethylolbutanal mixture formed. This crude TMP does not containany alkali metal or alkaline earth metal formates or other impuritieswhich are formed in the inorganic Cannizzaro process. Likewise, thecrude TMP contains only small amounts, from about 5 to 10 mol %, oftrialkylammonium formates or free trialkylamine, unlike the productobtained from the organic Cannizzaro process.

[0017] The crude TMP which comes from the hydrogenation and is to besubjected to the purification process of the present invention comprisestrimethylolpropane and water together with methanol, trialkylamine,trialkylammonium formate, longer-chain linear and branched alcohols anddiols, for example methylbutanol or ethylpropanediol, addition productsof formaldehyde and methanol onto trimethylolpropane, acetals such asdimethylolbutyraldehyde TMP acetal and di-TMP.

[0018] Good results are obtained using crude hydrogenation productscomprising from 10 to 40% by weight of trimethylolpropane, from 0 to 10%by weight of 2,2-dimethylolbutanal, from 0.5 to 5% by weight ofmethanol, from 0 to 6% by weight of methylbutanol, from 1 to 10% byweight of trialkylammonium formate, from 0 to 5% by weight of2-ethylpropanediol, from 0.1 to 10% by weight of high boilers such asdi-TMP or other addition products and from 5 to 80% by weight of water.Crude hydrogenation products having such a composition can be obtained,for example, by the process described in WO 98/28253. Before thepurification of the present invention to decompose the trialkylammoniumformate-, the crude hydrogenation product can firstly be worked up bycontinuous distillation as described in examples 2 and 3 of DE-A-199 63435. However, the purification according to the present invention of thecrude hydrogenation products is preferably carried out without priortreatment by distillation.

[0019] Catalysts used in the process of the present invention areheterogeneous catalysts comprising at least one metal of groups 8 to 12of the Periodic Table, for example ruthenium, osmium, iridium, platinum,palladium, rhodium, iron, copper, cobalt, nickel and zinc and alsocombinations of these metals. These metals can be used either in theform of the pure metals or in the form of their compounds, for exampleoxides or sulfides. Preference is given to using copper, nickel, cobalt,ruthenium or palladium catalysts. These catalysts can have been appliedto the customary-supports, for example TiO₂, Al₂O₃, ZrO₂, SiO₂, carbonor mixtures thereof. The supported catalysts obtained in this way canalso be in the form of all known shaped bodies. Examples are extrudatesor pellets.

[0020] The use of supported copper-, nickel- and/or cobalt-containingcatalysts is preferred.

[0021] Raney copper, Raney nickel and Raney cobalt catalysts aresuitable for use in the process of the present invention. These Raneycatalysts can be in the form of all known shaped bodies, for examplepellets, extrudates or granules. Suitable Raney copper catalysts are,for example, the Raney copper catalysts in the form of nuggets which areknown from WO 99/03801, which is hereby expressly incorporated byreference. These catalysts have a particle size of the nuggets of from 2to 7 mm, a copper content of from 40 to 90% by weight, a surface areadetermined by the Langmuir method of from 5 to 50 m²/g, a copper surfacearea of from 0.5 to 7 m²/g, an Hg pore volume of from 0.01 to 0.12 ml/gand a mean pore diameter of from 50 to 300 nm.

[0022] EP-A-672 452, which is hereby expressly incorporated byreference, discloses suitable nickel-containing catalysts which comprisefrom 65 to 80% of nickel, calculated as nickel oxide, from 10 to 25% ofsilicon, calculated as silicon dioxide, from 2 to 10% by weight ofzirconium, calculated as zirconium oxide, and from 0 to 10% by weight ofaluminum, calculated as aluminum oxide, with the proviso that the sum ofthe contents of silicon dioxide and aluminum oxide is at least 15%(percentages quoted are by weight and are based on the total mass of thecatalyst), and are obtainable by addition of an acidic aqueous solutionof nickel, zirconium and, if desired, aluminum salts to a basic aqueoussolution of silicon and, if desired, aluminum compounds, with the pHbeing reduced to at most 6.5 and subsequently being adjusted to from 7to 8 by addition of further basic solution, isolation of the solid whichhas been precipitated this way, drying, shaping and calcination.

[0023] Furthermore, the hydrogenation catalysts known from EP-A-044 444,which is hereby expressly incorporated by reference, which have aspecific surface area of from 50 to 150 m²/g, which have, eitherentirely or partly, a spinel structure, in which copper is present inthe form of copper oxide and in whose preparation copper and aluminumare precipitated from their compounds in a ratio of from 0.25 to 3 atomsof copper to one atom of aluminum in the presence of carbonates at a pHof from 4.5 to 9 and the precipitate obtained in this way is calcined atfrom 300 to 800° C.

[0024] The zirconium-, copper-, cobalt- and nickel-containing catalystknown from DE-A 198 26 396, which is free of oxygen-containing compoundsof molybdenum, is also suitable for use in the process of the presentinvention.

[0025] In a particularly preferred embodiment, the purificationaccording to the present invention is carried out in the presence of thecatalyst known from DE-A 198 09 418, which is hereby expresslyincorporated by reference, which comprises an inorganic support in whichTiO₂ is present and, as active component, copper or a mixture of copperwith at least one metal selected from the group consisting of zinc,aluminum, cerium, noble metals and metals of transition group VIII andwhose specific copper surface area is not more than 10 m²/g. Thesecatalysts preferably have TiO₂ or a mixture of TiO₂ and Al₂O₃ or amixture of TiO₂ and ZrO₂ or a mixture of TiO₂, Al₂O₃ and ZrO₂ assupport; particular preference is given to using TiO₂. In thepreparation of this catalyst as described in DE-A 19809418, metallic Cupowder can be added as further additive during tableting so that thecopper surface area is not more than 10 m²/g.

[0026] In a particular embodiment of the invention, the decomposition ofthe trialkylammonium formate present in the crude TMP is carried outover a catalyst suitable for the hydrogenation of the precursor of theTMP (2,2-dimethylolbutanal), for example over the copper catalyst knownfrom DE-A-198 09 418.

[0027] This embodiment is particularly economical since thedecomposition of the trialkylammonium formate can occur in thehydrogenation reactor of the hydrogenation process as described in WO98/28253 and only one catalyst is required. The decomposition productsof the trialkylammonium formate, viz. CO and water and/or CO₂ andhydrogen, can be removed from the reactor via the offgas line. However,the decomposition of the trialkylammonium formates according to theprocess of the present invention can likewise be carried out in aseparate reactor.

[0028] In the process of the present invention, the decomposition of thetrialkylammonium formates is generally carried out at from 100 to 250°C., preferably from 140 to 220° C. The pressures used are generallyabove 10⁶ Pa, preferably in the range from 2×10⁶ to 15×10⁶ Pa.

[0029] The process of the present invention can be carried out eithercontinuously or batchwise, with preference being given to a continuousprocess.

[0030] In a continuous process, the amount of crude trimethylolalkanefrom the hydrogenation process or the organic Cannizzaro process ispreferably from about 0.05 to about 3 kg per liter of catalyst per hour,more preferably from about 0.1 to about 1 kg per liter of catalyst perhour.

[0031] As hydrogenation gases, it is possible to use any gases whichcomprise free hydrogen and do not contain harmful amounts of catalystpoisons, for example CO. For example, it is possible to use offgasesfrom a reformer. Preference is given to using pure hydrogen.

[0032] The process of the present invention is illustrated by theexamples below.

EXAMPLES

[0033] Preparation of Crude TMP

[0034] An apparatus comprising two heatable stirred vessels connected toone another by means of overflow pipes and having a total capacity of 72l was supplied with fresh aqueous formaldehyde solution (4 300 g/l inthe form of a 40% strength aqueous solution) and n-butyraldehyde (1 800g/h) and with fresh trimethylamine as catalyst (130 g/h) in the form ofa 45% strength aqueous solution. The reactors were maintained at 40° C.

[0035] The output was fed directly into the upper part of a falling filmevaporator with superposed column (11 bar steam for heating) andfractionally distilled there under atmospheric pressure to give alow-boiling top product consisting essentially of n-butyraldehyde, ethylacrolein, formaldehyde, water and trimethylamine and a high-boilingbottom product.

[0036] The top product was condensed continuously and recirculated tothe above-described reactors.

[0037] The high-boiling bottom product from the evaporator (about 33.5kg/h) was admixed continuously with fresh trimethylamine catalyst (50g/h, in the form of a 45% strength aqueous solution) and introduced intoa heatable tube reactor which was provided with random packing and hadan empty volume of 12.1. The reactor was maintained at 40° C.

[0038] The output from the after-reactor was introduced continuouslyinto the upper part of a further distillation apparatus, viz. theformaldehyde removal (11 bar steam for heating), and fractionallydistilled there to give a low-boiling top product consisting essentiallyof ethyl acrolein, formaldehyde, water and trimethylamine and ahigh-boiling bottom product. The low-boiling top product (27 kg/h) wascondensed continuously and recirculated to the first stirred vessel,while the high-boiling bottom product was collected.

[0039] The bottom product obtained in this way consisted essentially ofwater together with dimethylol butyraldehyde, formaldehyde and traces ofmonomethylol butyraldehyde. It was then subjected to a continuoushydrogenation. For this purpose, the reaction solution was hydrogenatedat 90 bar and 115° C. in a main reactor operated in thecirculation/downflow mode and a downstream after-reactor operated in thecirculation mode. The catalyst was prepared by a method analogous tocatalyst J in DE 198 09 418. It comprises 40% of CuO, 20% of Cu and 40%of TiO₂. The apparatus used comprised a 10 m long heated main reactor(internal diameter: 27 mm) and a 5.3 m long heated after-reactor(internal diameter: 25 mm). The flow around the circuit was 25 l/h ofliquid, and the feed to the reactor was set to 4 kg/h. Accordingly, 4kg/h of hydrogenation product were taken off.

Examples 1 to 4

[0040] The TMP used has the composition 25% by weight of TMP, 0.35% byweight of 2,2-dimethylolbutanal, 0.53% by weight of methanol, 0.078% byweight of methylbutanol, 0.23% by weight of ethylpropanediol, 0.43% byweight of adducts of TMP with formaldehyde and methanol, 0.036% byweight of TMP formate, 0.46% by weight of TMP dimethylbutanal acetals,0.46% by weight of high boilers, 1.7% by weight of trimethylammoniumformate and 70% by weight of water. 180 ml of this crude solution weretreated with hydrogen at 180° C. and 90 bar in the presence of acatalyst as indicated in table 1 which had been prereduced at 180° C.and 25 bar. After one hour, the dimethylolbutanal content and the TMPcontent were determined by gas chromatography. The formate concentrationwas determined by means of titration with tetrabutylammonium hydroxide.The results obtained are summarized in table 1.

Comparative example 5

[0041] The procedure of examples 1 to 4 was repeated, but the reactionwas carried out in the absence of a catalyst. The result is shown in thetable. Amount DMB TMP Formate Formate Shaped of catalyst % by % by % byconversion No. Catalyst bodies [g] area¹ area¹ weight² [%] Startingmaterial 1.17 83.4 0.74 — 1 Cu/TiO₂ 3 × 3 mm 20 0 85.1 0.33 55 (DE 19809 418) pellets 2 Cu/Al₂O₃ 5 × 5 mm 9.5 0 85.2 0.40 46 (EP 0 044 444)pellets 3 Ni/SiO₂/Al₂O₃/ZrO₂ 1.5 mm 12.5 0 84.5 0.09 87 (EP 0672 452)extru- dates 4 Co/Ni/Cu/ZrO₂ 5 × 3 mm 20.5 0 85.1 0.31 58 (DE 198 26396) pellets 5 — — — 0 83.4 0.71 4

[0042] It can be seen from the table that ammonium formate can bedecomposed catalytically with high conversions at 180° C. over the aboveCu, Ni and Co catalysts. On the other hand, virtually no formateconversion is achieved under purely thermal conditions in thecomparative example. In addition, it is clear that the TMP yield isincreased by hydrogenation of DMB under the conditions according to thepresent invention.

1. A process for removing trialkylammonium formate from methylolalkaneswhich have been obtained by condensation of formaldehyde with a higheraldehyde, which comprises decomposing trialkylammonium formate atelevated temperature with the addition of a hydrogen-containing gas overcatalysts comprising at least one metal of groups 8 to 12 of thePeriodic Table.
 2. A process as claimed in claim 1, wherein supportedcopper-, nickel- and/or cobalt-containing catalysts are used.
 3. Aprocess as claimed in claim 1 or 2, wherein a catalyst suitable for thehydrogenation of 2,2-dimethylolbutanal is used.
 4. A process as claimedin any of claims 1 to 0.3, wherein trimethylolalkane obtained by thehydrogenation process is used.
 5. A process as claimed in any of claims1 to 4, wherein trimethylolpropane is used.
 6. A process as claimed inany of claims 1 to 5 carried out at from 100 to 250° C.
 7. A process asclaimed in any of claims 1 to 6 carried out at a pressure of from 2×10⁶to 15×10⁶ Pa.
 8. A process as claimed in any of claims 1 to 7 carriedout in the hydrogenation reactor of the hydrogenation process.